7.27 Spring 2006
Thought questions
1 of 3
1. The following information was posted on the WHO web site.
http://www.who.int/csr/don/2005_08_03/en/
Outbreak associated with Streptococcus suis in
pigs in China
3 August 2005
To date, the Ministry of Health in China has reported 206 cases of human
disease associated with an outbreak of Streptococcus suis in pigs. Of these
human cases, 38 have been fatal. As reported by China, 18 patients are
critically ill.
Virtually all cases have occurred in Sichuan Province, where infections with
Streptococcus suis have been detected in pigs in a concurrent outbreak. The
province has one of the largest pig populations in China.
Investigation and containment of the outbreak have been given high priority
by Chinese authorities. The country’s ministries of health and agriculture are
working in close collaboration, and WHO and FAO are being promptly
informed of new developments.
Investigations conducted by Chinese epidemiologists indicate that the first
human cases occurred at the end of June in Ziyang City, Sichuan Province.
From 24 June through 21 July, the authorities reported 20 cases of illness, of
unknown cause, admitted to three hospitals in that city. WHO was officially
informed of the outbreak on 22 July, at which time 20 cases and 9 deaths
had been reported.
Cases have since been reported in 11 prefectures in Sichuan Province. Most
cases reported have occurred in adult male farmers. Information reported to
WHO suggests that close contact with diseased or dead pigs is the principal
source of human infection.
Symptoms reported by local clinicians include high fever, malaise, nausea,
and vomiting, followed by meningitis, subcutaneous haemorrhage, toxic
shock, and coma in severe cases. The incubation period is short and disease
progression is rapid.
Local experts are conducting active searches for further cases. To date,
Chinese authorities say they have found no evidence of human-to-human
transmission.
The outbreak in humans has some unusual features and is being closely
followed by WHO. Diagnostic testing to further characterize the causative
agent is recommended as an essential part of ongoing efforts to understand
this outbreak, ensure its rapid containment, and prevent further deaths.
7.27 Spring 2006
Thought questions
2 of 3
a.
It appears that the strain of Streptococcus suis has involved in this
outbreak has acquired some new properties which make it a more
significant pathogen. What mechanisms can you suggest which might
lead to the rapid acquisition of such new pathogenic properties.
b.
What information might be obtained from sequencing the genome of
this pathogenic strain of Streptococcus suis which might give insight
into the pathogenic properties of the bacteria.
c.
Explain what type of gene the bacteria might have acquired which
leads to toxic shock. Explain the mechanism by which toxic shock is
induced by this gene
2. An infection is sweeping the MIT community which causes an upper
respiratory infection, general malaise and specifically attacks the math
centers in the brain causing a precipitous drop in test scores in all course 18
classes. A group of UROP students are determined to identify the cause of
this infection. They determine that the infection can be transferred to mice,
rabbits, cats and hamsters by swabbing a Q-TIP from the nasal secretions of
a student who has just failed an 18.01 exam on the tip of the nose of an
experimental animal. The same procedure fails to transfer any pathological
condition to dogs and guinea pigs.
a. Explain how the procedures students could use to determine what type of
infectious agent is responsible for the infection. Be specific about what
types of results would suggest the agents was a virus.
b. The infectious agent does turn out to be a virus. Explain how the students
might determine what type of virus they had isolated.
c. The students isolate three distinct proteins from the virus A, B and C. The
students divide themselves into 3 groups and attempt to make antibody
against these proteins. The first group has only enough of protein A to
inject a rabbit once, so they harvest serum after a single injection. The
second group has enough of protein B to inject a rabbit twice and then
they harvest the serum. The third group has enough serum to inject a
rabbit a total of times and then harvest the serum. What differences
would you expect to observe for the antiserum harvested by each group
of students. Explain the reasons for these differences in terms of the
molecular and cellular mechanisms for generating antibody.
d. Explain three different strategies the UROP students might use to attempt
to develop a vaccine against the virus. Explain which cell types of the
immune system would be involved in each approach to vaccination.
e. A similar outbreak has occurred at Harvard, but the symptoms differ
slightly. Although the math performance of the students appears
undiminished (measurements turn out to be statistically significant on this
point despite the relatively low baseline) the Harvard students uniformly
exhibit an inability to bs effortlessly as they had been able to do prior to
infection. These two pathological functions turn out to be encoded by
different viral genes. A meeting held at a bar in Central Square between
infected Harvard and MIT students leads to students becoming infected
with both types of viruses. What outcome would you expect if the virus
7.27 Spring 2006
Thought questions
3 of 3
turns out to be an influenza virus. How would you expect this virus to
behave if it were to infect BU students. Would your view of the outcome
of this meeting differ if the virus turns out to be a picorna virus.
f. If the virus was an influenza virus, what chemical treatments might be
effective in treating the disease. Explain the molecular basis for these
treatments.
g. What reasons could you suggest for the inability of the virus to infect dogs
and hamsters.
3. The term epitope refers to the part of an antigen which is specifically
active in eliciting an immune response. A virus encoding a single coat protein
is capable of eliciting both an antibody response and a cytotoxic T cell
response. Would you expect these responses to be directed towards the
same epitopes on the virus coat protein? If not, why not? Be specific in your
answer referring to the components of the immune system active in each
case and explain why the portion of the polypeptide chain against which the
response was directed would differ in each case.